1. Field of the Invention
The present invention relates to a digital device having superconductors and, more particularly, to single-flux-quantum digital device employing single quanta of magnetic flux.
2. Description of the Related Art
A Josephson-junction device having superconductors is known to be a digital device capable of fast operation at low power consumption. As shown in FIG. 5, the Josephson-junction device is formed by connecting two superconductors 21 by a thin insulating film 22. Cooper pairs that cause supercurrent pass the insulating film 22 due to the tunneling effect according to the quantum theory. The principle of operation of the Josephson-junction device is divided roughly into a voltage-state logic (latching logic) and a flux-state logic (single-flux-quantum logic).
The voltage-state logic uses the strong nonlinear current-voltage characteristic of the Josephson-junction device. As shown in FIG. 6, the current-voltage characteristics of the Josephson-junction device has a state where voltage if zero because Cooper pairs move by the tunneling effect. Referring to FIG. 6, when current increases from zero at the origin O, a supercurrent increases along a path Oa while voltage remains zero (superconducting state). Since there is a limit to the intensity of the supercurrent that flows through the Josephson-junction device, the supercurrent reaches a critical current I0 at the point A and the state switches suddenly to a point B for a voltage state. Thereafter, the current increases monotonously with the voltage to a point C. When the current is decreased from the point C, the current-voltage characteristic varies along a path CBDO and any zero-voltage state does not appear (hysteretic characteristic). Therefore, in the Josephson-junction device, two discrete logic states can be created by supplying a supercurrent below the critical current I0 to the Josephson-junction device in the initial state and temporarily increasing the current beyond the critical current I0 for transition from the initial state to the voltage state. Once the Josephson-junction device is set in the voltage state by an input signal, the Josephson-junction device remains in the voltage state and is unable to return to the superconducting state even if the input signal is removed (latching characteristic).
The flux-state logic is a logic using the nonlinear current-phase characteristic of the Josephson-junction device. The relation between supercurrent I and phase difference xcfx86 in the Josephson-junction device is expressed by:
I=I0 sin xcfx86xe2x80x83xe2x80x83(1)
where I0 is critical supercurrent and xcfx86 is the phase difference between the wave functions of the superconductors 21 in the opposite sides of the insulator 22.
Since there is a limit to the supercurrent that flows through the Josephson-junction device, the phase state changes instantly upon the increase of the supercurrent beyond the critical supercurrent I0. Therefore, in the Josephson-junction device, the two discrete logic states can be created by supplying a supercurrent below the critical current I0 to the Josephson-junction device in the initial state and temporarily increasing the current beyond the critical current I0 in response to an input signal for transition from the phase state.
Methods of temporarily increasing the supercurrent to the critical supercurrent I0 or above in response to an input signal to realize the voltage-state logic or the flux-state logic are divided into those of a field-controlled (flux coupling) driving system and those of a current-controlled (direct coupling) driving system. The method of the field-controlled driving system passes a current (input current) through a coil disposed near the Josephson-junction device to reduce the critical supercurrent of the Josephson-junction device by applying a magnetic field to the Josephson-junction device. The method of the current-controlled driving system passes a current (input current) directly through the Josephson-junction device to increase the current flowing through the Josephson-junction device to the critical supercurrent I0 or above.
In the Josephson-junction device of the voltage-state logic or the flux-state logic, the current flowing through the Josephson-junction device is increased temporarily to the critical supercurrent I0 or above in response to the input current by the method of the field-controlled or the current-controlled system to realize the two discrete logic states. However, driving of the Josephson-junction device by either the methods of the field-controlled system or the current-controlled system has the following drawbacks.
A first problem is that, since the Josephson-junction device is a two-terminal device, a special contrivance is necessary to isolate the input and the output from each other. This drawback is particularly significant when the Josephson-junction device is driven by the driving method of the current-controlled system that passes the input current directly through the Josephson-junction device and the Josephson-junction device must be provided with a special circuit, such as an isolator for isolating the input and the output from each other. The input and the output of the Josephson-junction device can be relatively easily isolated from each other when the Josephson-junction device is driven by the driving method of the field-controlled system because all the input current supplied to the coil flows into the ground. However, an additional space is necessary to install the coil and a neighboring Josephson-junction device must be widely spaced apart from the coil so that the neighboring Josephson-junction device may not be affected by the magnetic field created by the coil. Accordingly, it is difficult to construct an integrated circuit in a high level of integration when Josephson-junction devices are employed.
A second problem is that, since the input signal is a current, the Josephson-junction device needs an operating system different from that of the current semiconductor devices and the Josephson-junction device has problems associated with fan-out and load inductance.
A third problem is that, since the Josephson-junction device has a hysteretic current-voltage characteristic, the Josephson-junction device of the voltage-state logic must be returned to a zero-voltage state. Returning to the zero-voltage state can be achieved by a previously proposed method that uses an AC-driven circuit or a previously proposed method that uses a DC-driven circuit. These methods, however, causes many problems in implementation and circuit speed. A Josephson-junction device not having any hysteretic characteristic and using a RSFQ (rapid single-flux-quantum) logic has been proposed by Likharev et al. in 1991. However, even the RSFQ logic is unable to solve the first and the second problem.
The present invention has been made considering those problems and it is therefore an object of the present invention to solve the foregoing first to third problems and to provide a three-terminal single-flux-quantum digital device capable of raising the level of integration, dealing with input and output voltage signals, not having a hysteretic characteristic and capable of easily realizing circuit design similar to those of conventional semiconductor devices.
According to the present invention, a single-flux-quantum digital device (hereinafter referred to as xe2x80x9cSFQ digital devicexe2x80x9d) comprises a first superconducting line extended in a large ring, a second superconducting line connected to the first superconducting line so as to divide the large ring of the first superconducting line into two small rings, a switching device for regulating a supercurrent flowing through the second superconducting line, and a detecting device for detecting a change in the supercurrent flowing through the first superconducting line.
In the SFQ digital device according to the present invention, the switching device increases or decreases the supercurrent flowing through the second superconducting line to increase or decrease the supercurrent flowing through the first superconducting line. This change in the supercurrent is detected through the detection of a change in the output voltage of the detecting device to realize the basic logic of a logic circuit or a storage circuit by using the relation between the input and the output signal. Particularly, the SFQ digital device according to the present invention is a three-terminal device capable of raising the level of integration, and dealing with input and output voltage signals, not having a hysteretic characteristic and facilitating circuit designing similarly to the conventional semiconductor devices.